The man chosen by the pope to succeed Cardinal Roger Mahony as head of the most populous Roman Catholic archdiocese in the United States will officially begin his ministry in the Southland today during a gala Mass at the Cathedral of Our Lady of the Angels.

Coadjutor Archbishop Jose H. Gomez, head of the church in San Antonio since 2004, was chosen in April by Pope Benedict XVI to replace Mahony, who will retire on his 75th birthday on Feb. 27.

Today’s 2 p.m. Mass will mark the beginning of Gomez’s work in the Archdiocese of Los Angeles, which claims more than 5 million Catholics in Los Angeles, Ventura and Santa Barbara counties.

Gomez will serve as an assistant bishop until Mahony’s retirement.

A replacement for Gomez in San Antonio has not yet been named, and that diocese is being overseen by an interim administrator, according to Los Angeles Archdiocese spokesman Tod Tamberg.

Today’s Mass will will include the rite of reception. After Mahony gives his sermon, Sister Mary Elizabeth Galt, chancellor of the archdiocese, will read a letter from Rome announcing Gomez’s appointment as coadjutor archbishop. Mahony will accept the letter and ask the audience — expected to include about 4,000 people — if they will support Gomez, and they will reply “We are.”

While Mahony will be the principal celebrant, the Mass will include six other cardinals, 59 bishops and 411 priests.

Los Angeles native Cardinal William Levada, the highest-ranking American in the Vatican, will attend, as will Philadelphia Archbishop Justin Rigali, also a Los Angeles native.

Gomez, 58, will address the congregation after Communion, and he will greet and bless people on the Cathedral Plaza following the Mass.

Church rules require archbishops to tender their resignation at age 75. The pope can allow them to stay on, but he otherwise appoints a successor.

Gomez is a Monterrey, Mexico-born priest of the conservative Prelature of Opus Dei. He earned undergraduate degrees in accounting, philosophy and theology and was ordained in the Prelature of Opus Dei in 1978.

In 1980, he earned a doctorate in theology from the University of Navarre’s campus in Pamplona, Spain.

Gomez was the priest in residence at Our Lady of Grace Catholic Church in San Antonio from 1987 to 1999. During that period, he became a regional representative to the National Association of Hispanic Priests and, in 1995, was named president. In 1999, he became executive director.

As the archbishop in San Antonio, Gomez helped establish the Hispanic Seminary of Our Lady of Guadalupe in Mexico City, which opened in 2000.

In 2001, he was ordained an auxiliary bishop for the Archdiocese of Denver, where he served in several areas, including outreach to Hispanics. He also organized Denver’s Centro San Juan Diego for Family and Pastoral Care, a place for formation of lay leaders and a base to provide welcoming services to immigrants.